1. Field
The disclosures herein generally relate to semiconductor devices, and particularly relate to a high-power field-effect transistor using nitride semiconductor.
2. Description of the Related Art
Nitride semiconductors as typified by GaN, AlN, InN, and mixed crystals thereof have a large bandgap, and, for that reason, are utilized for a short-wavelength light emitting device. Since such nitride semiconductors having a large bandgap do not suffer breakdown even under high electrical field, the use of these semiconductors in application to high-power electronic devices has been attracting attention. Examples of such high-power electronic devices include a high-power field-effect transistor, especially a high-power HEMT.
Even for such a high-power electronic device utilizing nitride semiconductor, there has been a continuing effort to further reduce a gate leak current for the purpose of achieving an improved high-power performance.
FIG. 1 is a drawing showing the configuration of a high-power HEMT 10 using GaN as an electron transport layer according to the related art.
Referring to FIG. 1, the high-power HEMT 10 is formed on a semi-insulating SiC substrate 11. An electron transport layer 12 made of undoped GaN is epitaxially formed on the semi-insulating SIC substrate 11.
An electron supply layer 14 made of n-type AlGaN is epitaxially formed on the electron transport layer 12 with an undoped AlGaN spacer layer 13 intervening therebetween. Further, an n-type GaN layer 15 is epitaxially formed on the electron supply layer 14. In conjunction with the forming of the electron supply layer 14, 2-dimensional electron gas (2DEG) 12A is formed in the electron transport layer 12 over the interface with the spacer layer 13.
Further, a gate electrode 16 that includes an Ni electrode film 16A providing a schottky junction and a low-resistance Au film 16B stacked thereon is formed on the n-type GaN layer 15. Further, ohmic electrodes 17A and 17B including a Ti film and an Al film stacked one over the other are formed as a source electrode and a drain electrode, respectively, to be in direct contact with the electron supply layer 14 in such a manner as to be spaced apart from the gate electrode 16.
Moreover, a passivation film 18 made of SiN or the like is formed to cover the exposed surface of the n-type GaN layer 15. In the illustrated example, the passivation film 18 covers the ohmic electrodes 17A and 17B, and, also, is tightly attached to the sidewall surfaces of the gate electrode 16.
With the configuration described above, the electron supply layer 14 is covered by the n-type GaN layer 15 including no Al, so that the formation of interface state due to the oxidization of Al is suppressed on the surface of the electron supply layer 14. This serves to reduce a leak current propagating through the interface state, thereby making it possible to drive the HEMT 10 at high power.
In recent years, there has been a demand for the increased high-power driving of a high-power HEMT using a nitride semiconductor such as GaN. In order to meet such a demand, there is a need to further reduce the leak current generated in the high-power HEMT, especially a leak current generated between the gate and the drain.